Differentiating lung neuroendocrine neoplasms from tumor-like infection using CT in patients with ectopic ACTH syndrome

Objectives Pulmonary neuroendocrine neoplasms (NENs) are the most frequent cause of ectopic adrenocorticotropic hormone syndrome (EAS); lung infection is common in EAS. An imaging finding of infection in EAS patients can mimic NENs. This retrospective study investigated EAS-associated pulmonary imaging indicators. Methods Forty-five pulmonary NENs and 27 tumor-like infections from 59 EAS patients (45 NEN and 14 infection patients) were included. Clinical manifestations, CT features, 18F-FDG, or 68Ga-DOTATATE-PET/CT images and pathological results were collected. Results High-sensitivity C-reactive protein (p < 0.001) and expectoration occurrence (p = 0.04) were higher, and finger oxygen saturation (p = 0.01) was lower in the infection group than the NENs group. Higher-grade NENs were underrepresented in our cohort. Pulmonary NENs were solitary primary tumors, 80% of which were peripheral tumors. Overlying vessel sign and airway involvement were more frequent in the NENs group (p < 0.001). Multifocal (p = 0.001) and peripheral (p = 0.02) lesions, cavity (p < 0.001), spiculation (p = 0.01), pleural retraction (p < 0.001), connection to pulmonary veins (p = 0.02), and distal atelectasis or inflammatory exudation (p = 0.001) were more frequent in the infection group. The median CT value increment between the non-contrast and arterial phases was significantly higher in NENs lesions (p < 0.001). Receiver operating characteristic curve analysis indicated a moderate predictive ability at 48.3 HU of delta CT value (sensitivity, 95.0%; specificity, 54.1%). Conclusion Chest CT scans are valuable for localizing and characterizing pulmonary lesions in rare EAS, thereby enabling prompt differential diagnosis and treatment. Critical relevance statement Thin-slice CT images are valuable for the localization and identification of pulmonary ectopic adrenocorticotropic hormone syndrome lesions, leading to prompt differential diagnosis and effective treatment. Key Points Lung tumor-like infections can mimic neuroendocrine neoplasms (NENs) in ectopic adrenocorticotropic hormone syndrome (EAS) patients. NENs are solitary lesions, whereas infections are multiple peripheral pseudotumors each with identifying imaging findings. Typical CT signs aid in localization and creating an appropriate differential diagnosis. Graphical Abstract


Introduction
Ectopic adrenocorticotropic hormone (ACTH) syndrome (EAS) is a rare condition with an annual incidence of 0.8 per million [1].EAS accounts for 9-18% of ACTHdependent Cushing's syndrome (CS) and 5-10% of all CS cases [2,3].EAS is often associated with rapidly progressive clinical features such as severe hypokalaemia and high ACTH and cortisol levels [4].Neuroendocrine neoplasms (NENs) are a rare and heterogeneous group of tumors, encompassing well-differentiated neuroendocrine tumors (NETs) and poorly differentiated neuroendocrine carcinomas (NECs) [5,6].The common causes of ectopic ACTH production include NETs of the lungs and thymus, pancreatic NETs, pheochromocytomas, and medullary thyroid carcinomas [7].Lung NETs (LNETs) are the most frequent cause of EAS.Small cell lung carcinoma (SCLC) is another possible etiology of EAS [8].
Timely detection and resection of NETs are essential for good clinical outcomes.
The prevalence of systemic opportunistic infections due to hypercortisolism is higher (up to 51%) in EAS patients compared to patients with Cushing's disease (21%) [9,10].Infection most frequently involves the lungs [11,12] and is the leading cause of mortality within 90 days of CS treatment initiation, accounting for 31% of deaths and outpacing cardiovascular and cerebrovascular events [13].Pneumocystis jirovecii and species from the genera Aspergillus, Cryptococcus, and Nocardia are well-known pathogens in EAS patients [14][15][16].In some cases, pulmonary infections can appear as nodules or masses and mimic lung tumors, leading to unnecessary surgical treatment [12,17].
Therefore, discriminating LNETs from tumormimicking infection lesions is crucial.Diagnosis of EAS usually necessitates a combination of functional imaging methods such as somatostatin receptor scintigraphy, 18 F-fluorodeoxyglucose (FDG) or 68 Ga-DOTATATE PET/CT, dynamic observation, and experienced radiologists.Computed tomography (CT) is a widely used imaging method with high sensitivity for identifying pathologically confirmed EAS tumors (85.4%), especially those located in the chest cavity (91.2%) [4].Additionally, thin-slice CT combined with three-dimensional (3D) postprocessing techniques can show the anatomical relationship between LNETs and airways more clearly.However, some LNETs are very small, occult, or lack specific CT imaging features.
Due to its rarity, the CT features of pulmonary NENs causing EAS have not been systematically elucidated in a large patient population.Therefore, this retrospective study comprehensively evaluated the clinical and imaging features of 59 confirmed EAS cases, involving one group of patients with lung NENs and another group of patients with pulmonary infections.This study also identified CT features that may aid in distinguishing lung NENs from tumor-like pulmonary infections for better clinical management of EAS.

Patient selection
This retrospective study involved 59 EAS patients admitted to our hospital from March 2010 to March 2022.A computerized search of EAS was conducted using the keywords "Ectopic Cushing's Syndrome", "Ectopic Adrenocorticotropic Hormone Syndrome", and "Ectopic Corticotropin Releasing Hormone Syndrome" in the hospital information system.Then, the clinical records and radiological images were carefully reviewed and validated by a senior endocrinologist (L.L., with 25 years of experience in endocrinology) and a senior chest radiologist (L.S., with 18 years of experience in pulmonary imaging diagnosis).This study was approved by the Institutional Review Board of our hospital (I-23PJ175), and written informed consent was waived.
The diagnosis of EAS was based on biological, radiological, and pathological results (Fig. S1).The inclusion and exclusion criteria are listed in the Supplementary Appendix 1.1.
Based on the inclusion and exclusion criteria, 45 EAS patients with pulmonary NENs (45 lesions) and 14 EAS patients with pulmonary infections (27 lesions) were included in this study (Fig. 1).

Clinical information collection
The baseline clinical data from before treatment were obtained from electronic medical records (Table E1).We retrospectively collected clinical features, biochemical indicators of hypercortisolism (serum ACTH, plasma, and urinary cortisol), infection indicators (such as routine blood tests, high-sensitivity C-reactive protein [hs-CRP],

CT image acquisition and evaluation
The latest CT scans of each patient before surgery or etiology confirmation were used for evaluation.All chest CT examinations were performed from the lung apex to the lung base using various sets of multidetector CT scanners.In addition to non-contrast CT (NCCT), 37 (82.2%)pulmonary NEN patients and nine (64.3%) infection patients also underwent contrast-enhanced CT (CECT).The CECT scans were acquired at 35 s after the injection of 80-100 mL of nonionic contrast material (Ultravist 300, Bayer Schering Pharma AG, Germany) intravenously at a rate of 2.5 mL/s.The details of the scanning parameters and reconstruction methods are provided in Table E2.
CT images of the pulmonary lesions were evaluated according to the published literature [18,19].Each CT scan was reviewed on thin-slice CT images (slice thickness ≤ 1 mm) with mediastinal (width: 450 HU, level: 50 HU) and lung (width: 1200 HU, level: −600 HU) window settings using a PACS (GE Healthcare, Chicago, USA).Evaluation was carried out by two senior chest radiologists (L.S. and W.S.) with 18 and 29 years of experience in pulmonary imaging diagnosis, respectively, who were blinded to pathological and pathogenetic testing results.The two readings were performed on the same patient independently on the same day, and differences were resolved through consensus.CECT images were also evaluated if available.The CT morphological features of each lesion and adjacent pulmonary abnormalities were assessed.The involvement of the airway was also evaluated using coronal, sagittal, three-dimensional, and minimum intensity projection (MinIP) reconstruction images, when necessary.A detailed introduction and definitions of the CT features are described in the Supplementary Appendix 1.3 and Table E1.The changes in lung lesions were evaluated when serial chest CT scans were available.The quantitative measurements from the two radiologists were averaged, and the consensus on each qualitative evaluation was documented.

Functional imaging results
The results of somatostatin receptor scintigraphy, 18 F-FDG or 68 Ga-DOTATATE-PET/CT were collected if available and were confirmed by an experienced nuclear medicine physician (Z.Z., with 28 years of experience in nuclear medicine imaging diagnosis).The detailed protocols are described in Supplementary Appendix 1.4-1.6.

Statistical analysis
Data were expressed as the mean ± standard deviation (SD) or median (Q25, Q75).Chi-square or Fisher's exact tests, Student's t-test, Mann-Whitney U tests and oneway ANOVA were used when appropriate.Cut-off values, areas under the curve (AUCs) and sensitivity and specificity values were obtained by receiver operating characteristic (ROC) analysis.All statistical analyses were performed using SPSS 26.0 (SPSS Inc., Chicago, IL, USA).p < 0.05 was considered statistically significant.

Patient demographics and clinical features
This study included 72 pulmonary lesions from 59 EAS patients.Clinical features and laboratory results are summarized in Tables 1, 2 and Table E3.Forty-five EAS patients with confirmed pulmonary NENs were diagnosed at baseline (28/45, 62.2%) or during follow-up (17/45, 37.8%) with solitary primary NENs, comprising 32 G1 NETs (71.1%), nine G2 NETs (20.0%) and four SCLCs (8.9%).Among 14 pulmonary infection patients with EAS, only two thymus NENs were found to cause EAS, while the other 12 (85.7%)retained a diagnosis of occult EAS even after follow-up.Lower finger oxygen saturation (95.0% vs. 98.0%, p = 0.01) and higher occurrence of expectoration (21.4% vs. 2.3%, p = 0.04) were observed in patients with infection compared to those with NENs (Table 1).The hs-CRP was 17.8-fold higher in the infection group than in the NEN group (14.2 mg/L vs. 0.8 mg/ L, p < 0.001; Table E3).Additionally, most clinical manifestations and biochemical EAS results such as serum cortisol, ACTH, and 24-hour urinary free cortisol (UFC) levels did not significantly differ between the two groups (Tables 1 and 2, all p > 0.05).

Discussion
In this study, we summarized the clinical and radiological manifestations of ectopic adrenocorticotropic hormone syndrome in 45 patients with pulmonary neuroendocrine neoplasms and 14 patients with tumor-like pulmonary infections.To our knowledge, this is the first study to comprehensively analyze the CT imaging features of this rare disease in such a large sample size.The findings revealed that CT features can help effectively distinguish between neuroendocrine neoplasms and tumor-like infections in ectopic adrenocorticotropic hormone syndrome patients.Most biochemical results, such as ACTH, cortisol, and other inflammatory indicators were not significantly different between the two groups.Infection in hypercortisolism may lack typical manifestations of infectious disease such as high temperature and white blood cell count [20].Limited laboratory evidence of infection highlights the significance of radiological methods, particularly CT scan, in the differential diagnosis of EAS.
Chest CT scan is easily accessible and routinely used to diagnose EAS, due to the high prevalence of EAS originating in the lung.Our study showed that a chest CT scan was useful for identifying EAS lesions and distinguishing NENs from infections.First, multifocal peripheral lesions, cavity, spiculation, and pleural retraction on CT images indicate a pseudotumor-like infection in EAS cases.Opportunistic infections by Nocardia species and fungi tend to form cavities.Xu et al reported that cavity (6/12, 50%), consolidation/infiltration (5/12, 42%), and nodule/ mass (4/12, 33%) were frequent imaging features in EAS with Nocardia infection [21].Another study reported similar findings, stating that cavity (8/16) was the most common finding, followed by nodules (7/16), infiltration (3/16), and consolidation (2/16) [22].Second, overlying vessel sign and airway involvement were more common in EAS NENs, which may be correlated with the abundant vessels and a common airway origin in well-differentiated NETs [23,24].Third, most NENs presented with moderate-to-intense enhancement, higher than the infection lesions, and had a moderate capacity to be distinguished (AUC = 0.774).This may be attributed to the large number of NETs (carcinoid) in this cohort (71.1% NET G1 and 20% NET G2) and the high vascularity of carcinoids leading to significant enhancement on CECT [25,26].High-grade NENs generally have a higher incidence compared to their low-grade counterparts [5].However, in EAS cases, lower-grade NENs are more frequently observed [3,10,27].Few SCLC cases with EAS were diagnosed, possibly due to the severity of the primary disease obscuring the features of CS.Although CT findings may differ, histopathological confirmation is still necessary in cases with EAS and pulmonary masses.Additionally, 80% of EAS NENs had a tumor size less than 2 cm in our study, which may be attributed to the application of thin-slice CT reconstruction [28] and early diagnosis due to severe hypercortisolism [29].The tumor size of NENs leading to EAS (mean: 1.1 cm, range: 0.4-3.0cm) [30] was reported to be smaller than nonfunctioning lung carcinoids (median: 2.1 cm, range: 0.7-9.0cm) [31], which is consistent with our study (median [Q1, Q3], 1.1 [0.9, 1.7] cm).EAS NETs show distinct, more aggressive features compared with hormone-quiescent carcinoids, but the mechanism has not been clearly elucidated [30].
PET/CT is commonly used for localizing the source of EAS, however the sensitivity of 68 Ga-DOTATATE PET/ CT (46.2%) was lower than 18 F-FDG PET/CT (70.0%) in our patients with NENs.Four EAS patients were false negative in both 18 F-FDG and 68 Ga-DOTATATE PET/ CT, while CT successfully detected all small pulmonary NEN nodules.These were resected afterward with excellent postoperative results and complete resolution of symptoms.Thin-slice chest CT imaging offers better detection of small EAS lesions during breath-hold scans, whereas PET/CT performed during free breathing may cause respiratory motion artifacts and hamper the detectability of small lung lesions.The false negative rates in PET/CT may be attributed to well-differentiated EAS tumors with low uptake [32].In EAS localization to the lungs, a previous study showed comparable sensitivity between 68 Ga-SSTR PET/CT and CT [79.4% (7/9) vs 77.8% (77/97)] [32].Another study reviewed 68 Ga-PET/ CT results in 69 EAS cases and revealed a slightly lower sensitivity compared to CT (64.0%vs 69.7%) [33].False negative scans from 68 Ga-SSTR PET/CT may be underreported.Ceccato et al also reported an EAS patient that was negative on 68 Ga-SSTR-PET/CT but positive on 18 F-FDG PET, which is likely attributed to tumor cell dedifferentiation and diminished expression of somatostatin receptors [34].High cortisol level downregulates SSTRs expression, also influencing the accuracy of 68 Ga-SSTR-PET/CT [35].No significant elevation was observed in 24-h UFC in our double-negative PET/CT patients, but their tumor size tended to be smaller.A previous study consistently reported slightly smaller lesions (12 mm vs 16 mm, p = 0.26) in the 68 Ga-SSTR PET/CT negative group [33].
Tumor-like infection in EAS can exhibit high uptake in PET/CT, leading to misdiagnosis as NENs.Hou et al reported a higher FDG uptake in pulmonary infectious lesions compared to EAS lung tumors (mean SUV max : 5.9 vs. 2.1, p = 0.01) [36].In our study, the median SUV max of 18 F-FDG-PET/CT was also higher in pulmonary infections than in NENs, but with a borderline difference (2.4 vs. 1.3, p = 0.05).This discrepancy may be due to the different subtypes of infections and NENs.The majority of NENs in our study were NET G1 tumors, usually presenting with low FDG uptake [37].Cryptococcus and Candida lesions had a high SUV max (over 4.95), while lesions of Nocardia, Aspergillus, and other fungi presented with moderate FDG uptake.Cryptococcus nodules have been reported to exhibit increased 18 F-FDG PET/CT uptake (SUV max = 13.6)[38,39].Therefore, functional imaging methods combined with CT scan are needed for precise diagnosis.
Our study had some limitations.First, the study population might be biased due to the rarity of EAS and the low availability of pulmonary infection cases occurring simultaneously.Second, this study was retrospective, and CECT scanning was conducted in only 78% of the patients.In addition, there is limited 68 Ga-DOTATATE PET/CT data because this method has only been in use since 2012.Third, the patient numbers corresponding to each pathogen were relatively small; thus, larger sample sizes are required for further study.
In conclusion, this study comprehensively summarized the typical CT features of pulmonary neuroendocrine neoplasms and tumor-like infections in ectopic adrenocorticotropic hormone syndrome.Thin-slice CT images can provide a vivid evaluation of lesions and their relationship to adjacent lung tissues, such as small bronchi and vessels.Therefore, they can be valuable for localizing ACTH-secreting tumors, more prompt differential diagnosis, and effective treatment of EAS.

Fig. 1
Fig. 1 Flowchart of patient eligibility with inclusion and exclusion criteria.EAS Ectopic adrenocorticotropic hormone syndrome, NEN Neuroendocrine neoplasm

Fig. 2 Fig. 3 Fig. 4
Fig. 2 Representative CT images and schematics of pulmonary abnormalities of pulmonary neuroendocrine neoplasms in patients with ectopic ACTH syndrome.A Solitary solid nodule in the right middle lobe.B Solitary solid nodule in the left upper lobe.C Central lesion adjacent to the left hilar region.D Solid nodule with moderate enhancement.E Solid nodule related closely to adjacent broncho-vascular bundles.F Solid nodule with overlying vessel sign.G Solid nodule with a clear boundary, no cavity, spiculation, and pleural retraction sign.H Occult micro-nodule mimic a cross-section of blood vessels.I Solid nodule with subsegmental bronchi involved (yellow arrow) and overlying vessel sign (red arrow).J Solid nodule with subsegmental bronchi involved

Fig. 5
Fig. 5 Tumor-like pulmonary infection in a 34-year-old woman with confirmed ectopic Cushing's syndrome (CS) (A-D).Whole body 18 F-FDG PET/CT scan showing two fluorodeoxyglucose-avid pulmonary lesions in the right lower lobe (RLL) with a maximal standard uptake (SUV max ) up to 12.4 (big lesion) and 5.7 (small lesion) and no other abnormalities.Technetium-99 m somatostatin receptor scintigraphy also showed slightly increased uptake in the pulmonary mass and nodule in the RLL.A Axial and (B) coronal non-contrast chest CT at the lung window showing a mass (arrows, diameter 3.1 cm) in the RLL and a nodule (orange arrow in B, diameter 1.1 cm) beside the right oblique fissure.A, C, D An air bronchogram, lobulation, and spiculation were present in the mass.C Non-contrast and (D) contrast-enhanced axial CT at the mediastinal window showing homogeneous weak enhancement of about 15 HU increments (arrows).B, D A thick-wall cavity in the mass and (A) pleural retraction is visible.The two lesions were surgically removed and misdiagnosed as ACTH-secreting lung neuroendocrine tumors.Histopathological results confirmed pulmonary cryptococcosis with ACTH negativity, based on immunohistochemistry.The CS symptoms and laboratory tests were not resolved following surgery

Fig. 6
Fig. 6 Follow-up CT scans to locate a possible ACTH-secreting tumor in a 48-year-old man with ectopic Cushing's syndrome (A-D), The lung nodule was pathologically confirmed as typical carcinoid.No abnormality was found in the somatostatin receptor scintigraphy.Both 18 F-FDG PET/CT and 68 Ga-DOTATATE PET/CT did not show increased uptake in the small nodule in the left lower lobe (LLL).Chest CT showing a small peripheral solid nodule (diameter 0.9 cm) in the LLL (arrows in A-C).A Axial and (B) coronal 1 mm thin-slice non-contrast CT images at the lung window showing a nodule with a clear boundary and lobulated shape.C Thin-slice axial contrast-enhanced CT at the mediastinal window showing moderate homogeneous enhancement (arrow, Δ CT value = 54 HU after contrast injection).D The micro solid nodule (diameter 0.4 cm) in the LLL (arrow) was misdiagnosed as a cross-section of a blood vessel on the baseline CT five years prior.The misdiagnosis may have been caused by hindsight bias, poor image quality influenced by a slight respiratory motion artifact, and the 5 mm thick-slice thickness College, Beijing 100730, China. 34+4 Medical Doctor Program, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing 100730, China. 4Department of Endocrinology, Key Laboratory of Endocrinology of National Health Commission, Peking Union Medical College Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing 100730, China. 5Department of Nuclear Medicine, State Key Laboratory of Complex Severe and Rare Diseases, Peking Union Medical College Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing 100730, China. 6Department of Pathology, Peking Union Medical College Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing 100730, China.

Table 1
Patient demographics and clinical features between EAS with pulmonary NENs and infection

Table 3
Patient-level morphological features and lesion location on chest CT between EAS with pulmonary NENs and infection * The lesions, including primary tumors, intrapulmonary metastases, and intrapulmonary lymph node # The NEN group has 46 tumors with a case involved multiple lobes

Table 4
Lesion-level CT imaging features between EAS with pulmonary NENs and infection a Delta CT value: CT value increment after contrast enhancement b An intrapulmonary lymph node was observed intraoperatively and on CT (with pathology confirmed)